1. Field of the Invention
The present invention relates to a transfer apparatus and an image forming apparatus which form images by an electrophotographic method using developing agent transferred onto a transfer material (for example, a paper) with a transfer belt.
2. Description of Related Art
In recent years, demand has increased to form a full-color image as well as a monochromatic image by an image forming apparatus of electrophotographic type, and such an electrophotographic full-color image forming apparatus is under development. Normally, the full-color image forming apparatus forms images using color toner (developing agent) corresponding to each image data of a plurality of colors decomposed from a color image. For example, the same color image is read through each of the filters of each color (red, green, blue) of the three primary colors for additive color mixture, and an image data of each color (cyan, magenta, yellow) of at least the three primary colors for subtractive color mixture is created from the read data. Based on the image data of each color, a visible image is generated using toner of the corresponding color, and these visible images of the respective colors are superposed one on another thereby to form a full-color image.
In this full-color image forming apparatus, the exposure process, the development process and the transfer process are required for each color, while at the same time occurring the problem of aligning the visible images of the respective colors in position. In view of this situation, the rate at which the full-color image is formed is apparently considered lower than the rate at which the monochromatic image is formed. To overcome this problem, a full-color image forming apparatus of tandem type has conventionally been proposed in which a plurality of image forming units for forming visible images of different colors are arranged in line on the outer peripheral surface of a rotatable semiconductive endless belt along the direction of movement thereof, so that a full color image may be formed before the endless belt makes at least one rotation.
To increase the speed of forming a full-color image, the full-color image forming apparatus of tandem type employs an intermediate transfer method in which the visible images of the respective colors formed in the image forming units are superposed one on another on the outer peripheral surface of the endless belt and then transferred onto the paper, or a transfer conveyance method in which the visible images of the respective colors formed by the image forming units are transferred sequentially onto the surface of a transfer material (for example, a paper) conveyed by adsorption on the outer peripheral surface of the endless belt (for example, Japanese Patent Application Laid-Open No. 10-039651 (1998) and Japanese Patent Application Laid-Open No. 10-293437 (1998) and Japanese Patent No. 2574804).
FIG. 1 is a schematic diagram for explaining the configuration of the essential portion of the conventional full-color image forming apparatus employing the intermediate transfer method. The full-color image forming apparatus shown in FIG. 1 comprises an image forming unit 200 of electrophotographic type, in which a full-color image is formed on the paper through a primary transfer process for transferring the toner images of the respective colors in superposed relation with each other on a transfer belt 201 and a secondary transfer process for transferring onto the paper the multi-color toner image formed on the transfer belt 201 in the primary transfer process. The transfer belt 201 is configured to move along the direction of the white arrow by a transfer belt driving roller 202 and a transfer belt driven roller 203. Photosensitive drums 204a through 204d corresponding to the respective colors (for example, yellow, magenta, cyan and black) and intermediate transfer rollers 205a through 205d in opposed relation to the photosensitive drums 204a through 204d, respectively, are arranged along the path of the transfer belt 201.
In such conventional full-color image forming apparatus, consider a case in which an image is printed based on the image data inputted from the external. First, the electrically charged toner images of the respective colors are formed on the surface of the respective photosensitive drums 204a through 204d. Then, high-voltage transfer bias is applied to the intermediate transfer rollers 205a through 205d, so that the toner images on the photosensitive drums 204a through 204d are sequentially transferred onto the transfer belt 201. In the process, the transfer timing of the respective toner images is controlled, so that the toner images of the respective colors are superposed one on another and a single multi-color toner image is formed on the transfer belt 201. Then, a high voltage of opposite polarity to the charge polarity of the toner is applied to the transfer roller 206 arranged in the subsequent stage of the primary transfer process, with the result that a multi-color toner image is transferred on the paper supplied from a paper feeding unit 210. The paper onto which the multi-color toner image has been transferred is conveyed to a fixing unit-not shown, where the multi-color toner image is fixed on the paper. Thus, a printed matter formed with a full-color image is completed.
The toner image transfer efficiency of the full-color image forming apparatus of intermediate transfer type described above is determined by the toner transfer field and the adherence between the toner and the transfer belt 201. The toner transfer field is controllable by adjusting the transfer current, the transfer voltage and the transfer nip. By optimizing these parameters and thus improving the transfer efficiency, the image quality can be improved. On the other hand, the adherence between the toner and the transfer belt 201 is the intermolecular attraction (Van der Waals force) between them, and therefore dependent on the material of the transfer belt 201 and the shape of the toner on the one hand and varies with the contact area between the toner and the transfer belt 201 at the same time.
FIG. 2 is a schematic diagram showing the state of the toner attached on the transfer belt 201. In the case where the contact pressure between the photosensitive drum 204a and the transfer belt 201 is small, as shown in FIG. 2A, the contact area between the transfer belt 201 and the toner transferred in the primary transfer process is reduced, and so are both the intermolecular attraction between them and the cohesion between the toner. As a result, the electrostatic transfer in the secondary transfer process is facilitated, and the toner image can be satisfactorily transferred onto the paper. In the case where the contact pressure between the photosensitive drum 204a and the transfer belt 201 is large, on the other hand, as shown in FIG. 2B, the contact area between the toner and the transfer belt 201 is increased. Therefore, the intermolecular attraction between them and the cohesion between the toner are both increased, so that the electrostatic transfer of the toner image in the secondary transfer process becomes difficult. Especially in the full-color image forming apparatus of tandem type described above, the toner images corresponding to the respective colors are superposed sequentially on the transfer belt 201, and therefore, the cohesion between the toner is promoted with the progress of the transfer process, thereby causing the reduced transfer efficiency of toner image transfer from the transfer belt 201 onto the paper. Such reduction in transfer efficiency due to the intermolecular attraction between the toner and the transfer belt 201 cannot be easily improved by electrostatic control. It is necessary, therefore, to appropriately adjust the contact pressure between the photosensitive drums 204a through 204d and the transfer belt 201 and thereby to improve the transfer efficiency and the image quality.
When sequentially transferring the toner images of the respective colors to the transfer belt 201, the charge potential of the toner attached on the transfer belt 201 increases gradually. Therefore, the voltage applied to the intermediate transfer rollers 205a through 205d is required to be increased progressively. To differentiate the voltage applied to the intermediate transfer rollers 205a through 205d, however, a high-voltage transformer, etc. is required, thereby occurring the problem of the apparatus becoming bulky and an increased production cost.